This study proposes to examine the efficacy of a public health dose (PHD) of exercise as an augmentation strategy in the treatment of Major Depressive Disorder (MDD). We will assess patients on SSRI treatment who still have significant residual symptoms of depression, defined by a Hamilton Rating Scale for Depression (17-item; HRSD17) score >14. Incomplete response with significant residual symptoms is common in MDD and is associated with an increased likelihood of relapse and poorer quality of life and function. Augmentation strategies are often implemented to alleviate residual symptoms. However, limitations of pharmacological augmentation, such as lack of overwhelming evidence for efficacy for most agents and increased risks of adverse side effects, often render this strategy undesirable. Exercise may be a viable augmenting strategy for the treatment of residual symptoms of depression since it is associated with a reduced risk of side effects, as well as many associated benefits, including increased longevity and reduced risks of many common medical conditions. Our recently completed randomized controlled trial, DOSE (Depression Outcomes Study of Exercise), revealed that a public health dose of exercise significantly reduced symptoms of depression, measured by the HRSD17, compared to a low dose of exercise or flexibility exercise control group. A logical next step in our investigation of the use of exercise in the treatment of depression is to assess its viability as an augmenting strategy in persons who responded to SSRI therapy, but still endorse significant residual depressive symptoms. The goal of our exercise intervention will be remission, defined as an HRSD17 score of 7 or less. Patients meeting DSM-IV criteria for Major Depressive Disorder, who have been taking an SSRI at an adequate dose for 8-12 weeks, will be randomized to a PHD or low dose of exercise for 12 weeks. The HRSD17 will serve as the primary outcome measure. In addition, the 30-item versions of the Inventory for Depressive Symptomatology, Clinician-Rated (IDS-C30) and Self-Report (IDS-SR30) will be assessed. Quality of life and satisfaction will also be evaluated. The primary aims of the study are to: 1) compare the effectiveness of PHD exercise augmentation to low dose augmentation in eliciting remission in responders to SSRI treatment who have significant residual symptoms of depression; 2) to compare functioning and quality of life between treatment groups; 3) to examine the extent to which symptom reduction mediates improvement in social function; and 4) to explore the pattern of symptom reduction that occurs over the course of exercise augmentation.